1. Field of the Invention
The present invention relates to the preparation of a polydiorganosiloxane having silanol end groups by a polycondensation reaction.
2. Description of the Prior Art:
It is well known to this art to polycondense silanols attached to silanes and/or diorganopolysiloxanes in the presence of miscellaneous compounds which accelerate the polycondensation by serving as dehydrating agents or catalysts (compare the text by Walter Noll, Chemistry and Technology of Silicones, 1968 English language edition, Academic Press, pages 209 to 218).
It is also known, from U.S. Pat. No. 2,961,245, to carry out bulk polymerization by ring-opening of cyclotrisiloxanes containing fluorinated hydrocarbon radicals (1), in the presence of a perfluoroalkane sulfonic acid (3) and a polydiorganosiloxane having triorganosiloxy end groups (2) (principally hexamethyldisiloxane) as chain-blocking agent.
A fluorinated silicone oil is thereby obtained after devolatilization, the viscosity of which is mainly regulated by the ratio of the siloxane (2) to the siloxane (1), the catalyst (3) optionally being removed by distillation or washing.
U.S. Pat. No. 3,308,203 describes the polycondensation, in an organic solvent, of a mixture of linear and branched chain polyorganosiloxanes bearing silanol groups, in the presence of a catalyst which can be, inter alia, a perfluoroalkanesulfonic acid or p-toluene sulfonic acid. The solvent must be removed from the reaction mixture and, in cases where p-toluene sulfonic acid is used (Example 4), it is deactivated by adding LiOH and the salt formed is removed by filtration.
British Patent No. 1 325,654 describes the polymerization, in the presence of a perfluoroalkane sulfonic acid and silica, of cyclic polysiloxanes optionally mixed with chain-blocking linear diorganopolysiloxanes. Upon completion of the reaction, the catalyst can be neutralized with hexamethyldisilazane.
U.S. Pat. No. 4,448,927 describes the polymerization, in the presence of a filler material (silica or carbon black), of linear siloxane oligomers HO(R.sub.2 SiO).sub.n H and/or cyclic siloxane oligomers (R.sub.2 SiO).sub.y capable of containing, in addition, chain-blocking polydiorganosiloxanes. In Example 3, the polymerization is described of polydiorganopolysiloxane having silanol end groups in the presence of a filler and trifluoromethanesulfonic acid, deactivated upon completion of the reaction with MgO.
Similar processes of polymerization on filler material are also described in European Patents EP-A No. 119,816 and EP-A No. 119,817.
Finally, in European Patent EP-A No. 133,975, the polycondensation is described, in solvent medium, of polydiorganosiloxanes containing linear and branched silanol groups, using a catalytic system consisting of a free acid including, in particular, a perfluoroalkanesulfonic acid, and its amine salt.